1. Field of the Invention
This invention relates to a radiation image storage panel and more particularly to a radiation image storage panel comprising a support, a phosphor layer which comprise a binder and a stimulable phosphor dispersed therein, and a protective film, superposed in this order.
2. Description of the Prior Art
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt material and a radiographic intensifying screen.
As a method replacing the above-described radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968 has been recently paid much attention. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; exciting the stimulable phosphor with an electromagnetic wave such as visible light and infrared rays (hereinafter referred to as "stimulating rays") to sequentially release the radiation energy stored in the stimulable phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing a radiation image as a visible image from the electric signals. In the above-described radiation image recording and reproducing method, a radiation image can be obtained with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the case of utilizing the conventional radiography. Accordingly, this radiation image recording and reproducing method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the above-described radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (i.e., stimulated emission) when exposed to an electromagnetic wave such as visible light or infrared rays after having been exposed to a radiation such as X-rays. In the radiation image recording and reproducing method, the radiation having passed through an object or having radiated from an object is absorbed by the phosphor layer of the radiation image storage panel in proportion to the applied radiation dose, and a radiation image of the object is recorded on the radiation image storage panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by irradiating the panel with an electromagnetic wave such as visible light or infrared rays (i.e., stimulating rays). The stimulated emission is then photoelectrically detected to obtain electric signals, so as to reproduce a visible image from the electric signals.
It is required that the radiation image storage panel has a sufficient mechanical strength so as not to allow easy separation of the phosphor layer from the support as well as from the protective film, when the panel receives mechanical shocks and mechanical force in the course of possible falling or bending thereof. Further, since the radiation image storage panel hardly deteriorates upon exposure to a radiation or to an electromagnetic wave ranging from visible light to infrared rays, the panel can be empolyed repeatedly for a long period of time. Accordingly, it is necessary for the panel in the repeated use not to cause such troubles as the separation between the phosphor layer and the support and the separation between the phosphor layer and the protective film caused by mechanical shocks applied in the handling of the panel in a procedure of exposure to a radiation, in a procedure of reproducing a radiation image brought about by excitation with an electromagnetic wave after the exposure to the radiation, or in a procedure of erasure of the radiation image information remaining in the panel.
However, the radiation image storage panel has a tendency that the bonding strength between the support and the phosphor layer or the bonding strength between the protective film and the phosphor layer decreases as the mixing ratio of the binder to the stimulable phosphor (binder/stimulable phosphor) in the phosphor layer in contact with the support or protective film decreases, in other words, as the amount of the stimulable phosphor contained therein increases.
On the other hand, the radiation image storage panel generally provides an image of decreased sharpness, as the mixing ratio of the binder to the stimulable phosphor in the phosphor layer of the panel increases, in other words, as the amount of the stimulable phosphor contained in the phosphor layer decreases.
For these reasons, it is difficult to adjust the mixing ratio of the binder to the phosphor in the phosphor layer so as to satisfy not only the bonding strength between the support and the phosphor layer as well as that between the protective film and the phosphor layer, but also the sharpness of the image provided by the panel. As for the conventional radiation image storage panel having a single phosphor layer, there is hardly obtained a panel capable of providing an image of high quality as well as showing a sufficient bonding strength between the support and the phosphor layer and that between the protective film and the phosphor layer.